1. Field of the Invention
The present invention relates to an optical recording apparatus and an optical recording/reproducing apparatus, and particularly relates to an optical recording apparatus for recording information in an optical recording medium as a hologram and an optical recording/reproducing apparatus for recording information in an optical recording medium as a hologram and for reproducing the recorded information.
2. Description of the Related Art
Generally, an incident angle for diffracting light must be more precise as a hologram becomes thicker. The diffracted light disappears even if the incident angle is slightly shifted from Bragg condition. In a volume holographic memory, an angle multiplexing method utilizes this selectivity of the angle. That is, a plurality of holograms are formed in the same volume in such a manner that the angle of reference light is changed by two-beam interference in which interference fringes obtained by crossing signal light and the reference light are recorded. An arbitrary hologram can be read without a crosstalk by irradiation of reading light at the same incident angle as that of the reference light used in recording. However, it is a problem that the hologram recording by the two-beam interference is susceptible to vibration, and the recording must be carried out while the recording medium is stopped. There are other problems that an optical system becomes complicated and a drive apparatus is enlarged and the like.
Recently, “polarized collinear recording” has been proposed in order to overcome the problems in the two-beam interference, and has become commercially practical (Japanese Patent Application Laid-Open (JP-A) No. 11-311938). The “polarized collinear recording” is a method for recording the hologram, in which the reference light and the signal light are coaxially arranged and the recording medium is irradiated with the reference light and the signal light by using one objective lens. According to the method, using a servo technology can improve stability of recording/reproducing against the vibration, and miniaturization of the apparatus can be achieved.
The recording method of the “polarized collinear recording” will be described below referring to FIG. 4. P-polarized light, which is emitted from a light source device 125, is incident to a spatial light modulator 118 though a prism block 119, and the light is spatially modulated to become signal light. The signal light passes through a prism block 115 and is incident to a two-split azimuth rotator plate 114. Azimuth rotator plates 114L and 114R are provided in the two-split azimuth rotator plate 114. A polarization direction of the light, which has passed through the azimuth rotator plate 114L, is rotated by +45° to become A-polarized light, and a polarization direction of the light, which has passed through the azimuth rotator plate 114R, is rotated by −45° to become B-polarized light. The signal light which has passed through the two-split azimuth rotator plate 114 is condensed by an objective lens 112, and an optical information recording medium 101 is irradiated with the signal light.
On the other hand, in the light incident to a phase spatial light modulator 117 from the prism block 119, the phase is spatially modulated to become reference light for recording. The reference light for recording passes through a convex lens 116 and the prism block 115 and is incident to the two-split azimuth rotator plate 114. The reference light for recording which has passed through the two-split azimuth rotator plate 114 is condensed by the objective lens 112, and the optical information recording medium 101 is irradiated with the reference light for recording.
In a hologram layer 103 of the optical information recording medium 101, the signal light which is reflected by a reflection coating 105, having the same polarization direction, interferes with the reference light for recording proceeding toward the reflection coating 105 to form an interference pattern, and the interference pattern is recorded in the same volume.
The reproducing method of the “polarized collinear recording” will be described below referring to FIG. 5. P-polarized light, which is emitted from the light source device 125, is incident to the phase spatial light modulator 117 though the prism block 119, and the phase of the light is spatially modulated to become reference light for reproducing. The reference light for reproducing passes through the convex lens 116 and the prism block 115 and is incident to the two-split azimuth rotator plate 114. The polarization direction of the reference light for reproducing which has passed through the two-split azimuth rotator plate 114 is rotated and condensed by the objective lens 112, and the optical information recording medium 101 is irradiated with the reference light for reproducing.
When the optical information recording medium 101 is irradiated with the reference light for reproducing, reproducing light corresponding to the signal light in the recording is generated. The reproducing light passes through the objective lens 112 and the two-split azimuth rotator plate 114 to become S-polarized light. The reproducing light which has passed through the two-split azimuth rotator plate 114 passes through the prism block 115 and the spatial light modulator 118, and part of light quantity of the reproducing light is reflected by a half-reflecting surface 119b of the prism block 119 and detected by a CCD array 120.
However, the “polarized collinear recording” has the following disadvantages. 1) The reference light is reflected to follow the same light path for the reproducing light, the reference light can not be separated by a polarizing beam splitter, and a noise is generated in the reproducing. 2) The light path of the signal light does not completely coincide with that of the reference light, and the apparatus is still susceptible to vibration.
In view of the foregoing, it is an object of the present invention to provide an optical recording apparatus having excellent resistance against vibration. It is another object of the present invention to provide an optical recording/reproducing apparatus having excellent resistance against vibration and excellent resistance against noise.